Three dimensional woven fabrics have application in fibre-reinforced composite materials that are increasingly used as structural and other components in various industries such as, for instance, aerospace, automotive and construction. Such materials comprise a matrix of a suitable polymer such as, for example, epoxy that is reinforced with woven fibres such as, for example, carbon, glass, aramid or Kevlar. Their popularity is attributable to their light weight, high strength, thermal resistance, and ability to be formed into different shapes. Global sales of carbon fibre reinforced plastics, in particular, are forecast to increase dramatically in the next few years.
A three dimensional woven fabric typically comprises multiple layers of warp and weft yarns and vertical binder yarns for binding the layers together. One of the first examples of the manufacture of such fabrics was disclosed in U.S. Pat. No. 3,818,951 (Greenwood). The warp yarns are arranged vertical layers (each layer comprising a horizontal array of warp yarns) to provide the fabric thickness, adjacent layers being separated (shedded) in sequence for the insertion of a weft yarn. The shedding process is repeated down the layers until all the warp yarns are interspersed in the vertical direction with a corresponding weft yarn. A vertical binder yarn is inserted using a heald frame after each group of weft yarns has been inserted. This process is slow and the repeated shedding action causes damage and strain to the fibres. The method is therefore only suitable for a modest number of layers.
U.S. Pat. No. 3,834,424 (Fukuta) describes a method and apparatus for manufacturing a three dimensional fabric in which multiple weft yarns are simultaneously inserted into the sheds defined between the layers of warp. A stack of vertically spaced picking plates move from one side of the fabric to the other where the weft yarns are secured by a selvage yarns and vertical yarns are inserted from below and above. The plates then return to their original position. All three yarns are mutually orthogonal and are woven to create a rectangular or square block of fabric. The method does not allow for variations in fabric thickness and therefore does not provide for woven structures having differing cross-sections.
U.S. Pat. No. 5,085,252 (Mohamed et al.) discloses a method for three dimensional weaving that allows for the production of fabrics with non-rectangular cross-sections. It uses differential weft insertion from both sides. This allows for different lengths of weft insertion from one or both sides and therefore for woven structures having varying cross-sections. Heald frames are used to insert vertical binder yarns. The warp, weft and binder yarns are mutually orthogonal.
There is a continuing demand in the composite materials industry for three dimensional woven fabrics with stronger structural forms and more complex shapes but they must be produced in a cost-effective manner and with limited damage to the fibres. More complex woven fabric shapes are typically produced by joining together separate performs but this is undesirable as it adds another step to the manufacturing process and introduces weaknesses in the integrity and strength of the finished product. Many looms for producing three dimensional woven fabrics are complex. This renders them expensive and difficult to install, operate and maintain.
Three dimensional woven preforms are currently produced on conventional (single weft insertion) weaving machines equipped with a dobby or Jacquard shedding mechanism. These machines can perform a variety of weave styles including orthogonal, angle interlocked, layer-to-layer. These machines are however limited in their function as they a) require repeated movement of all the warp tows resulting in fibre damage; b) are limited in terms of the number of fabric layers that can be encompassed and hence are limited in terms of the thickness of the preform that can be produced; and c) one pick is inserted at a time and hence manufacturing is slow. Thus, methods of preparing three dimensional woven preforms using such machines is not ideal.
Multi-insertion three dimensional weaving processes on the other hand have the advantage of faster production due to multiple weft insertion, most of the warp (stuffer) yarns do not move and they can produce thicker preforms. However, only orthogonal weave is produced on conventional three dimensional weaving machines.
It is one object of the present invention to obviate or mitigate the aforesaid disadvantages.
It is an alternative object of the present invention to provide for an improved or alternative three dimensional woven fabric.